Cochlear implants have been applied to thousands of patients, with various degrees of success. That variety may be related to the numbers and locations of excitable cells that remain in each patient's auditory nerve. This project is designed to develop the means by which stimuli can be focused at specific volumes of the tissues of the inner ear and determine the geometry of the ear precisely. The first goal will be achieved by applying two electrical models of the inner ear to stimulus design. The first model, a lumped element model, is used to determine how to apply specific currents to specific electrodes in an array to produce a particular distribution of potentials in the implanted ear. The other model, a finite element model, is used to mimic in detail the potential distributions in the inner ear for specific stimuli. It verifies the validity of the simpler, and more computationally rapid, lumped-element model. This two-tiered approach to stimulus selection will be validated with physiological and psychophysical measurements in guinea pigs and monkeys. The second goal will be achieved with a novel histological technique that renders the bones of inner ear transparent. The structures are visualized with a fluorescence optical sectioning technique, and reconstructed in three-dimensional models. Those models will be analyzed for geometry and predictability of location from external landmarks. These goals focus on the solution of a specific problem in implant design: the direction of stimulus current to specific, excitable cells.